The actual guts of the algorithm are in compiled javascript

https://github.com/imeckler/diffgeo/blob/master/Native/ODE.j...

However, this seems to simply be using the numeric library.

http://www.numericjs.com/documentation.html

Which uses 'Dormand-Prince RK'.

https://en.wikipedia.org/wiki/Dormand%E2%80%93Prince_method